1. Field of the Invention
This invention relates to solenoid control circuits which are particularly useful in logging wells during drilling.
2. The Prior Art
The advantages of logging while drilling have long been recognized. However, the lack of a practical telemetering system has been a major obstacle to successful logging while drilling.
Of the various systems considered for logging while drilling, the most promising uses a solenoid-operated valve to create negative pressure pulses in the drilling fluid circulated through the drill string, drill bit, and borehole annulus. The negative pressure pulses are generated by intermittently opening and closing the valve to bypass a relatively small proportion of the total drilling fluid flow around the drill bit.
U.S. Pat. No. 4,078,620 shows a system which vents drilling fluid from the drill string interior to the annular space between the drill string and the borehole of the well to send negative pulses to the surface in a coded sequence corresponding to a sensed downhole condition. The Oil and Gas Journal, June 12, 1978, at page 71, discloses a similar system.
A general problem with using pressure pulses in the drilling fluid to send information is that the pulse generators to date have been bulky and, therefore, impose a wasteful pressure drop in the drilling fluid flowing through the drill string. The solenoid requires that the pulse generator be a certain minimum size because the solenoid must be large enough to supply the force needed to operate the valve, and it should require as little power as possible to extend operating time when only battery power is available. Even if an electrical generator is used downhole, the current required by the solenoid should be a minimum to permit reduction of solenoid size, and to avoid overheating, which is a serious problem in deep wells where downhole temperatures may exceed 300.degree. F.
This invention provides an efficient solenoid control circuit which permits the use of a relatively small solenoid, low operating power, and the design of a "slim" pulse generator to minimize energy losses due to drilling pressure drop in the drill string, and to permit the use of drill collars with a small outside diameter for deep drilling.
The solenoid circuit of this invention supplies a large opening current, followed by a smaller holding current to operate the valve and generate negative pressure pulses in a safe, efficient, and reliable manner. The switch control circuitry of FIGS. 8-10 in part is the subject of copending patent application Ser. No. 169,764 filed concurrently herewith.